Cell Structure and Organization

All living organisms are formed by cells, either one or several of them. Despite that there can be differentiation between them, all cells have at least the following components:

1. Bilipidic membrane: Two layers of phospholipids in 'toe-to-toe' arrangement. each phospholipid is made up of a glyceride (usually three carbon 'sugar'), fatty acids (hydrocarbon chains), and phosphate groups. Elements: C, O, H, P (some N). It is a 70-A wide flexible layer that keeps the inside (cytosol) of the cell separated from the surroundings. Proteins intercalated in the membrane regulate what goes in and out, and when it goes in and out.

2. Cell envelope: Surrounds the membrane. It is made up of polysaccharides (polymeric sugars, C, O, H) and can be anywere from 30 to 250 A thick. It gives the cell physical strenght, and protection from the surroundings, including other cells and larger multicellular organisms that may decide to gobble it up for dinner.

3. DNA/RNA: Composed of two (three) types of blocks - Nucleic acids (A, G, C, T, U), sugars (ribose and deoxy-ribose), and phosphate linkers (C, H, O, N, P). Their lenght varies, but they are ~ 20 A wide.

4. Ribosomes: Bundles of  ~ 50 proteins and RNA. Proteins are synthesized here using the data that comes on the messenger RNA. They are approximately 20 nm in diameter.

5. Proteins: made up of aminoacids (C, O, N, H, S), following the order specified by DNA fragments (genes). They pretty much do everything: They are in charge of creating energy (catabolism - energy production), synthesizing building blocks, other proteins, lipids, polysaccharides (anabolism - energy consumption), pumping stuff in and out of the cell, moving the cell around, etc., etc., (many more 'etcs.'). 1 to 10 nm in diameters, but they vary quite a bit (some are huge).

6. Cytoplasm: Everything else inside the cell - Water, salts, floating 'blocks' that need to be reused, etc., etc..

70% - Water
15% - Proteins
1% - DNA
6% - RNA
3% - Sugars
2% - Lipids
1% - Other orgnic stuff
1% - Inorganic ions

a) Prokaryotes are the simplest cells. 1 to 10 micrometers. They have only one mambrane (the outer one). Their DNA is coiled up in the cytoplasm, in what is called the NUCLEOID.

There are also flagella and pili. The first ones are little 'motors' that help the cell move around when looking for food, good spots, girlfriend/boyfriend, etc., etc. The latter helps the cell to attach to other cells - i. e., E. Coli in the intestine.

All bacteria are prokaryotes. Escherichia Coli (E. Coli) is a prokaryote, best known as the Guinea pig of molecular biologists.

b) Eukaryotes are larger than prokaryotes (10 to 100 micro-meters), and are far more complex. Eukaryotes can be organisms on their own, but they also are the cells found in all plants and animals. Appart from the outside membrane and cell wall, we also have a bunch of membrane-delimited organelles inside:

Rough endoplasmic reticulum & smooth endoplasmic reticulum: The two are responsible for protein and lipid synthesis, respectively. In the rough endoplasmic reticulum, ribosomes attached to them do the protein synthesis...

Golgi Complex: Proteins made in the rough endoplasmic reticulum & smooth endoplasmic reticulum are modified in the Golgi complex for 'final release'. The final location (membrane bound, cytoplasm) of the protein is also decided here.

Lysosome: Spherical vesicles that contain hydrolyzing enzymes. These are used by the cell to break down and 'digest' chunks of proteins, polysaccharides, etc., that may be ingested (see below). They function as a recyclying center.

Nucleus: They contain the DNA and genetic information that the cell requires to make up everything else (including thenucleus). It is composed of different membranes (the nuclear envelope), nucleoplasm, and a nucleolus (where the DNA actually piles up - chromatids). There we also have RNA (needed in transcription), and proteins (histones).

Mitochondria (mitochondrium): It has its own DNA and RNA, ribosomes, divides on its own, and makes some of its proteins and lipids. Working theory: mitochodria were prokaryotes that lived in simbiosis with larger cells (early eukaryotes) that were 'eaten' by them. The mitochondria are the 'energy cells' of cells, in which ATP is produced from oxidation reactions (burning up) of nutrients.

In plant cells we would also have chloroplasts, in which energy from the Sun is converted to chemical energy in a process known as photosysntesis, and a cell wall, which is pretty rigid and made up of cellulose.

Now, surrounding all that we would have large assemblies of proteins (actin and tubulin) which act as the skeleton of the cell, which, duh, is called the cytoskeleton. It should not be viewed as a rigid structure, but as very dynamic assemblied that change shape depending on the environmental conditions and stages of cell division. Also, macroscopic muscle contractions is due to the movement of actin fibers...

Apart from permeation through the membrane, euakaryotes can 'eat' stuff that may be floating around them. The cell membrane buds out, engulfs the stuff to eat, and brings it in. The lysosymes membranes then fuse with the cell membrane, materials are borken down, and get into the cell cytoplasm. ENDOCYTOSIS

Also, the budding can be outwards. This is how cells go to the bathroom... EXOCYTOSIS.

Now lets think of dimenssions. If PTC 132 was a cell, the relative sizes of everything else would be:

1. Thickness of outer cell membrane: like a thick plastic bag.

2. Cell wall: the thickness of a sweatshirt.

3. Mitocondria, Golgi complex, Nucleus, lysosome, etc.: would range from the size of a person to the size of a handbag (not in that order...).

4. Ribosome: The size of the eraser tip at the end of the pencil.

5. DNA strand: the thickness of a human hair and of varied lenghts. A fully extended chromosome would be pretty long, up to hundreds of miles!.

6. Mid-size protein: the size of sugar or salt grain.

7. Amino acids, sugar monomers, etc.: the smallest particles of dust visible to the naked eye that you can see floating around.

8. Water: You would not be able to see it with the naked eye, even at this scale!

Prepared by Randy J. Zauhar and Guillermo Moyna, 1999.